1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a hard disk drive having an actuator controller that adjusts seek current profile on the fly so as to improve seek performance.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost. However, because the computer industry continually strives to provide computer systems with increased performance, there exists a need for improved disk drives having increased data access speeds.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer over the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which generates a torque that moves the actuator arm. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The time required to reposition the transducer in the foregoing manner is known as the xe2x80x9cseek timexe2x80x9d of the drive and is an important performance factor that affects the throughput of the drive. For example, a drive having a short seek time will be able to access a requested track of data more quickly than a drive having a longer seek time. Currently, in high performance mass-market drives, the seek time required to reposition the transducer for a given distance of 0.8-0.85 cm is typically in the range of 5-10 ms.
In a typical seek operation, the transducer accelerates, coasts, and decelerates according to the predetermined control of the current applied to the VCM. The transducer, through a feedback control, typically requires some settling time to settle on the proper target track. Once the transducer is on the proper track, a track following current is provided to the VCM in order to maintain tracking.
To perform a seek operation, the current supplied to the VCM typically follows a predetermined profile that includes acceleration and deceleration phases. The profiles are typically stored in the controller, for example in a lookup table. The profiles are generally configured such that maximum acceleration and deceleration values leave ample margins between the values and maximum current that is available for use. One reason for having such margins is that the maximum current available for use by the VCM varies with the operating conditions and drive parameters of the disk drive. Such operating conditions may include factors such as temperature and supply voltage. The drive parameters may include driver FET resistance and VCM winding resistance. As is known in the art, operating conditions and drive parameters both affect how much current can be delivered to the VCM. Consequently, to avoid having the profiles exceeding the maximum available current, the profiles are generally configured in a conservative manner with ample margins that can accommodate a wide range of operating conditions and drive parameters. One method is to determine the worst case scenario of operating conditions and drive parameters, and formulate the worst case operating parameters accordingly.
One reason for attempting to have the current profile always within the available range is that seek operations that demand current beyond what is available may cause a situation where deceleration phase is not able to stop the transducer at the target track. In such a situation, the transducer overshoots the intended target, and a substantial amount of extra time is required to bring the transducer back and settle at the target track. Because of such a negative consequence, the deceleration profiles are generally configured to have ample reserve of current.
One disadvantage of utilizing conservative current profiles is that some seek time is sacrificed. In particular, the deceleration of the VCM, considered to be more important than the acceleration, is configured in a conservative manner because the available current is not known. Thus to decelerate the transducer using profile with conservative deceleration magnitude, the time required for deceleration needs to be extended.
To overcome such degradation in seek time in disk drives where certain specified seek time needs to be achieved for substantially all reasonable operating conditions, one solution is to use a high torque generating magnet to obtain a higher torque for a given VCM current. Such a high torque generating magnet may be implemented by using a magnet formed from high performance magnet materials, or by increasing the physical size of the magnet. While such a solution does achieve a specified seek time using the traditional conservative current profiles, both implementations of the high torque generating magnets are significantly more costly.
From the foregoing, it will be appreciated that there is a need for improved system for performing seek operations. To this end, there is a need for a system that utilizes the full range of available current to the VCM to form a current profile so as to improve the seek performance of the hard disk drive.
The aforementioned needs are satisfied by one aspect of the invention that relates to a hard disk drive comprising a rotatable disk having a magnetic recording media. The rotatable disk defines a plurality of concentric servo tracks. The hard disk drive further comprises a pivotable actuator that is movable with respect to the rotatable disk. The hard disk drive further comprises a transducer disposed on the actuator so as to be movable with respect to the disk so as to be positionable on a selected servo track of the plurality of concentric servo tracks. The hard disk drive further comprises a voice coil motor that moves the pivotable actuator in response to an applied current so as to permit movement of the transducer from a first location to a second location. The movement comprises an acceleration phase and a deceleration phase. The hard disk drive further comprises a controller for controlling the applied current according to a current profile. The current profile has a first portion corresponding to the acceleration phase, and a second portion corresponding to the deceleration phase. The controller monitors a parameter indicative of the movement of the actuator during the application of the first portion of the current profile to determine a performance related parameter. The controller then utilizes the performance related parameter to adjust the configuration of the second portion of the current profile to thereby reduce the time needed to position the transducer adjacent the second servo track.
In one embodiment, the performance related parameter is a peak acceleration current magnitude. In one specific case, the peak acceleration current is a saturation current whose magnitude depends on operating condition of the disk drive. The saturation current at a given operating condition defines a full range of current available for application to the voice coil motor. Determination of the saturation current magnitude permits adjustment of the current profile to utilize more of the full range of the available current. In another embodiment, the performance related parameter is an average acceleration current magnitude. In one specific case, the average accelerating current magnitude corresponds to an average saturated accelerating current.
In one embodiment, the parameter indicative of the movement of the actuator is the current being applied to the voice coil motor. In one specific case the current is measured directly by an analog to digital converter.
In one embodiment, the parameter indicative of the movement of the actuator is displacement of the transducer. The displacement of the transducer is determined by monitoring the servo wedges and track IDs encountered by the transducer. Rate of change of displacement of the transducer permits monitoring of velocity of the transducer. Rate of change of velocity of the transducer permits monitoring of acceleration of the transducer. The rate of change of velocity may also be determined by monitoring back-emf induced in the voice coil motor. The current being applied to the voice coil motor is inferred from the acceleration of the transducer. Such inference is possible because in one embodiment the acceleration of the transducer is linearly proportional to the current being applied to the voice coil motor.
In one embodiment, the parameter indicative of the movement of the actuator is displacement of the transducer during a predetermined time interval. The displacement of the transducer is correlated to a saturation point of a demanded digital to analog converter (DAC) value. The correlation between the displacement and the saturation DAC value is obtained from a lookup table. In one implementation, such a lookup table is determined during calibration of the disk drive.
In one embodiment, the movement of the transducer comprises a seek operation wherein the first location corresponds to a first servo track and the second location corresponds to a second servo track. The seek operation involves a seek time sufficiently long enough to establish a stable acceleration of the transducer. In one specific case, the sufficiently long enough seek time corresponds to seek lengths greater than approximately 200 servo tracks.
In one embodiment, the performance related parameter is determined during each seek operation. In another embodiment, the performance related parameter is determined periodically. In one specific implementation, the performance related parameter is determined after every 100 seek operations.
Another aspect of the invention relates to a method of adjusting a current profile on the fly during a movement of a transducer. The current profile determines the manner in which current is applied to a voice coil motor so as to move the transducer mounted on an actuator from a first location to a second location. The movement of the transducer comprises an acceleration phase followed by a deceleration phase. The method comprises applying an acceleration current according to the current profile so as to cause the acceleration of the transducer. The method further comprises determining the peak value of the acceleration current. The method further comprises adjusting the subsequent portion of the current profile based at least in part on the peak value of the acceleration current.
In one implementation, applying the acceleration current comprises applying a saturation current. Thus determining the peak value of the acceleration current comprise determining the magnitude of the saturation current. In one embodiment, the magnitude of the saturation current is determined by an analog to digital converter. In another embodiment, the magnitude of the saturation current is determined inferentially by monitoring the acceleration of the transducer. The acceleration of the transducer is determined from monitoring of displacement of the transducer. In yet another embodiment, the magnitude of the saturation current is determined by correlating the displacement of the transducer to the magnitude of the saturation current that caused the displacement. The correlation between the displacement and the magnitude of the saturation current is obtained from a lookup table.
In one implementation, adjusting the subsequent portion of the current profile comprises adjusting the profile of the deceleration current. Adjusting the profile of the deceleration current comprises increasing the magnitude of the deceleration current so as to utilize more of available current range as determined by the saturation current magnitude. Alternatively, adjusting the profile of the deceleration current comprises decreasing the magnitude of the deceleration current.
Yet another aspect of the invention relates to a method of adjusting a current profile that causes movement of a transducer. The current profile has a first portion corresponding to an acceleration phase and a second portion corresponding to a deceleration phase. The method comprises monitoring a parameter indicative of the movement of the transducer during application of the first portion of the current profile. The method further comprises determining a performance related parameter based at least in part on the parameter indicative of the movement of the transducer. The method further comprises adjusting the configuration of the second portion of the current profile based at least in part on the performance related parameter.
In one implementation, monitoring the parameter indicative of the movement of the transducer comprises monitoring a current that causes the movement of the transducer. In one specific case, monitoring the current comprises measuring the current""s magnitude using an analog to digital converter.
In another implementation, monitoring the parameter indicative of the movement of the transducer comprises monitoring the position of the transducer. Monitoring the parameter indicative of the movement of the transducer further comprises determining the velocity of the transducer based on the rate of change of the position of the transducer. Alternatively, the velocity of the transducer is determined by monitoring back-emf induced in the voice coil motor. Monitoring the parameter indicative of the movement of the transducer further comprises determining the acceleration of the transducer based on the rate of change of the velocity of the transducer. Monitoring the parameter indicative of the movement of the transducer further comprises determining the current that causes the movement of the transducer based on the acceleration of the transducer, wherein the current is inferred from the acceleration. The current is generally linearly proportional to the acceleration. Thus, determining the performance related parameter comprises determining the magnitude of the current at its peak value. In one specific case, determining the performance related parameter comprises determining the magnitude of a saturation current.
In yet another implementation, monitoring the current comprises monitoring the displacement of the transducer during a predetermined time interval and correlating the displacement to the saturation current magnitude that caused the displacement. The correlation between the displacement and the saturation current magnitude is obtained from a lookup table.
In yet another implementation, adjusting the configuration of the second portion of the current profile comprises adjusting the magnitude of peak value of the current of the second portion of the current profile based on the magnitude of peak value of the current of the first portion of the current profile. Adjusting the configuration of the second portion of the current profile comprises increasing the magnitude of the peak value of the current of the second portion of the current profile. Specifically, increasing the magnitude of the current of the second portion of the current profile comprises increasing the magnitude of the current of the deceleration phase. Alternatively, adjusting the configuration of the second portion of the current profile comprises decreasing the magnitude of the peak value of the current of the second portion of the current profile. Specifically, decreasing the magnitude of the current of the second portion of the current profile comprises decreasing the magnitude of the current of the deceleration phase. In one embodiment employing the various implementations of the method, the movement of the transducer is a seek operation.